1. Field of the Invention
The present invention relates to a distortion compensating device, in particular, to a distortion compensating device applicable to a high-frequency power amplifier for radio transmission for use in a wireless telephone apparatus, and a method for compensating distortion using the distortion compensating device.
2. Description of Related Art
Recently, as radio communication is being performed at high speed with largely increased capacity, linearity required by a power amplifier for radio transmission for use in a digital radio communication apparatus is becoming strict, which concurrently prevents improvement of power efficiency of a power amplifier.
On the other hand, consecutive talk time of widely spread digital wireless telephone apparatuses is being extended. Thus, in view of the marketing strategy of new digital radio communication apparatuses, putting an emphasis on extending the consecutive usage time cannot be ignored. So, the power efficiency of a power amplifier is being improved by employing a technique for compensating distortion.
However, this distortion compensating technique renders the circuit configuration enlarged, which makes it difficult to employ the technique in a wireless telephone apparatus which is required to be decreased in size and weight. Furthermore, due to the characteristics of a portable terminal, the environment to which a wireless telephone apparatus belongs will be varied significantly. Thus, the technique for compensating distortion should be that which can be adapted to the environment variation, which is an important subject as well as the miniaturization thereof.
There is disclosed xe2x80x9clinear transmission devicexe2x80x9d which employs an example of a conventional adaptive distortion compensating technique in the Japanese published patent number 2689011. FIG. 1 shows a block diagram of the configuration of the linear transmission device disclosed in the patent publication. As shown, the linear transmission device includes a power amplifier 59 which receives a modulated signal, a DC voltage converting circuit 55 for supplying a DC bias voltage to the power amplifier 59, and a directional coupler 52 and an envelope detecting circuit 53 for controlling the output voltage of the DC voltage converting circuit 55 in accordance with envelope signal level of the modulated signal received by the power amplifier 59. The input signal to be received by the power amplifier 59 is a modulated analog signal or a modulated digital signal, which is sent from a modulated signal input terminal 51. An amplified signal from the power amplifier 59 is to be output from an output terminal 62. A DC voltage terminal 56 supplies a DC voltage to the DC voltage converting circuit 55. The feature of the linear transmission device is that the signal to be received by the power amplifier 59 is controlled in accordance with the difference between the envelope signal level of the modulated signal and the envelope signal level of the output signal of the power amplifier 59. Thus, the linear transmission device further includes a directional coupler 60, an envelope detecting circuit 61, a differential signal generating circuit 54, a DC amplifier 57, and a power controlling circuit 58.
That is, the linear transmission device detects an envelope of the input signal and that of the output signal of the power amplifier 59, whose distortion should be compensated, by the envelope detecting circuit 53 and envelope detecting circuit 61, respectively. Then, the differential signal generating circuit. 54 generates a differential signal therebetween to send the signal to the DC amplifier 57, and the DC amplifier 57 amplifies the differential signal to generate a controlling signal and -sends the controlling signal to the power controlling circuit 58, and then the power controlling circuit 58 increases or decreases the level of the signal to be sent to the power amplifier 59. Thus, variation of gain of the power amplifier 59 due to temperature variation etc. is removed. Furthermore, since the DC voltage converting circuit 55 generates a voltage which varies in proportion to the envelope of the input signal and supplies the voltage to the power amplifier 59, the distortion of the power amplifier 59 can be compensated.
In performing above-described conventional distortion compensating technique, power controlling is performed by detecting the envelope of the input signal and that of the output signal and compensating the difference therebetween being the distortion. On the other hand, other distortion due to noise generated by the subtracting circuit (differential signal generating circuit 54) and amplifier (DC amplifier 57), and that due to nonlinearity can not be compensated, and thus generated distortion undesirably remains in the linear transmission, device.
It is therefore an object of the present invention to overcome the above mentioned drawbacks by providing a distortion compensating device which can significantly remove residual distortion, and a method for compensating distortion using the distortion compensating device.
According to the present invention, there is provided a distortion compensating device for compensating distortion generated at a device, including:
means for detecting an envelope voltage of an input signal supplied to the device;
means for generating an amplitude controlling signal adapted for controlling the amplitude of the input signal in accordance with the envelope voltage detected by the envelope voltage detecting means; and
means for controlling the amplitude of the input signal based on the amplitude controlling signal generated by the amplitude controlling signal generating means;
the distortion remaining in the output of the device being compensated based on the output of the amplitude controlling means.
Specifically, according to the present invention, the phase of distortion included in the output of the amplitude controlling means is rendered opposite to the phase of the distortion remaining in the output of the device.
According to the present invention, there is also provided a method for compensating distortion generated at a device, including the steps of:
detecting an envelope voltage of an input signal supplied to the device;
generating an amplitude controlling signal adapted for controlling the amplitude of the input signal in accordance with the envelope voltage detected at the envelope voltage detecting step;
controlling the amplitude of the input signal based on the amplitude controlling signal generated at the amplitude controlling signal generating step; and
compensating the distortion remaining in the output of the device based on the output generated at the amplitude controlling step.
According to the present invention, residual distortion can significantly be removed.
These objects and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.